The present invention provides an improved process for preparing bromine by reaction of chlorine with aqueous hydrobromic acid solutions.
The primary essential raw material for preparation of bromine is sea water. More than 90% of commercially manufactured bromine is prepared from open sea water having a bromine content from 60 to 70 mg/liter.
Concentrated solutions of hydrobromic acid are generally prepared from sea water according to the so-called "acid process" of Dow described in British Pat. No. 523,607 whereby the sea water is first acidified to a pH of about 3 to 4, then treated with sufficient chlorine to liberate bromine by the well-known reaction 2 NaBr + Cl.sub.2 .fwdarw. NaCl + Br.sub.2. The bromine formed is vaporized by aeration or by vacuum and then fixed, a preferred reaction for fixation being with sulfur dioxide according to the equation Br.sub.2 + SO.sub.2 + 2H.sub.2 O .fwdarw. 2HBr + H.sub.2 SO.sub.4. When a "sea water concentrate" is obtained in this manner, this concentrated bromide or hydrobromic acid solution, also identified herein as the "mother liquor", has a composition typically in the ranges of about 0.12 to 0.15 kilograms of hydrogen bromide per liter, about 0.005 to 0.01 kilograms of of hydrogen chloride per liter and about 0.09 to 0.11 kilograms of H.sub. 2 SO.sub.4 per liter. The pH of this mother liquor varies around zero.
Pure liquid bromine is conventionally obtained from this mother liquor by the action of chlorine in three stages.
In a first "prechlorination" step, the mother liquor is treated with only part of the amount stoichiometrically required to convert all the bromide therein to bromine, this first amount being limited to less than the amount which would result in separation of any liquid bromine, being thus no greater than sufficient to form a saturated solution of bromine in the mother liquor.
In a second "extraction" or "steaming-out" step, the prechlorinated mother liquor is simultaneously treated with the remainder of the chlorine required to convert the bromide to bromine, together with steam to effect steam distillation of the total bromine.
In a third "purification" step, the bromine condensed from the steam distillation is treated with an additional small amount of mother liquor, about 5 to 10% based on the initial weight used thereof.
Conventionally, it has not been considered practicable to prechlorinate with more than about two-thirds of the stoichiometric quantity of chlorine necessary to convert all the bromide in the mother liquor to bromine. Otherwise, liquid bromine separates out and contaminates the system. The process is aimed at keeping bromine in either the dissolved or the vapor state until it is condensed after the steaming-out tower. The prechlorination provides a head-start to making elemental bromine available, but the complete conversion by chlorine of bromide to bromine is not accomplished until the temperature of the reactants is around 100.degree.C in the steaming-out tower. At this point, it is the practice to use chlorine in an amount such that the total chlorine used is in slight excess of stoichiometric, about 1% in excess.
The operation of the process between the prechlorination and steaming-out steps has accordingly been directed toward strict avoidance of forming any liquid phase of bromine in the pipes connecting the two stages, particularly in the hydraulic guard or trap provided to keep the vapor phase of the steaming-out tower free of liquid bromine.
The steaming-out tower for the second conventional step operates in the vapor state at close to 100.degree.C. The stream from the prechlorination step, comprising a solution of bromine in hydrobromic acid mother liquor, enters the top of the steaming-out tower while the second charge of chlorine and the steam enter at the bottom. It is advantageous for the steaming-out tower to have a large capacity, exemplarily 3 or 4 times the volume of the prechlorination tower.
The mixture of water vapor and bromine vapor leaving the top of the steaming-out tower is then condensed and the liquid bromine and hot aqueous acid are separated, such as by decantation, from each other. The hot aqueous acid is recycled to the steaming-out tower.
The liquid bromine must be purified from small quantities of chlorine resulting from the use of an excess of chlorine to ensure complete conversion of all the bromide. The excess chlorine which is about 0.1 to 1% based on the weight of liberated bromine, must be eliminated to meet commercial specifications. This is accomplished in the third step of the prior-art procedure wherein the bromine is washed in a "purification tower" with exemplarily a 5 to 10% aliquot of the starting hydrobromic acid mother liquor.